The present invention relates to an electric power steering apparatus using a motor as a source of generating a steering assisting force.
Steering of an automobile is carried out by transmitting a rotational operation of a steering wheel, which is disposed in the inside of a car chamber, to a steering mechanism disposed on the outside of the car chamber for orienting the car wheels for steering (typically, front wheels).
FIG. 1 is a cross section view illustrating a construction of a conventional electric power steering apparatus. The electric power steering apparatus includes a first steering shaft 2 connected to a steering wheel 1, a torque sensor 5 that senses a steering torque by a relative shift amount, in the rotation direction, of the first steering shaft 2 to a second steering shaft 4 connected to the first steering shaft 2 via a torsion bar 3, and a speed reduction mechanism 9 having a worm 7 and a worm wheel 8 for reducing the speed of the output of a motor 6 for steering assistance, which is driven on the basis of the result of sensing by the torque sensor 5, and for transmitting the output to the second steering shaft 4. The electric power steering apparatus is constructed to assist the operation of the steering mechanism in accordance with the rotation of the steering wheel 1 with the rotation of the motor 6 so as to reduce the labor burden imposed on the driver for steering. The worm 7 is integrally formed at an axial middle part of a worm shaft 10 linked to an output shaft of the motor 6 and disposed to cross a shaft core of the second steering shaft 4.
FIG. 2 is an enlarged cross section view of a worm wheel in a conventional electric power steering apparatus disclosed in Japanese Utility Model Publication No. 2556890. The worm wheel 8 includes an annular tooth portion 81 made of synthetic resin and a boss portion 82 made of metal. The annular tooth portion 81 has teeth that engage with the worm 7, and the boss portion 82 is fitted to the inside peripheral surface of the annular tooth portion 81, so as to reduce the noise caused by engaging with the worm 7 and to improve the processability of the teeth. Further, the outside peripheral surface of the boss portion 82 has a concave 83 disposed at one axial end thereof, and the inside peripheral surface of the annular tooth portion 81 has a convex 84 disposed at one axial end thereof. The convex 84 is fitted into the concave 83.
Meanwhile, when the motor 6 assists steering, the worm wheel 8 rotates in the right direction or in the left direction and, since the worm wheel 8 has a tooth line twisted in the rotation direction relative to the rotation center line, a component of the torque transmitted from the worm 7 acts on the worm wheel 8 in the axial direction. Since the conventional worm wheel 8 is constructed in such a manner that the outside peripheral surface of the boss portion 82 has the concave 83 disposed merely at one axial end thereof and the inside peripheral surface of the annular tooth portion 81 has the convex 84 disposed merely at one axial end thereof to be fitted into the concave 83, a further enhancement of the fatigue strength of the fit portion is desired. Further, although the worm wheel 8 having the annular tooth portion 81 made of synthetic resin can have satisfactory heat-resistance, abrasion-resistance, and dimension-stability by selection of a synthetic resin material, a further enhancement of the strength of the dedendum, or tooth base, of the annular tooth portion 81 is desired.
In other words, since the annular tooth portion 81 of the worm wheel 8 is thermally deteriorated by frictional heat due to engaging with the worm 7 to decrease the strength of the dedendum and absorbs moisture in the ambient atmosphere to decrease the strength of the dedendum, resistance to such decrease in the strength of the dedendum caused by thermal deterioration and moisture absorption is demanded.
A principal object of the present invention is to provide an electric power steering apparatus that can further enhance the fatigue strength of the fit portion and the strength of the dedendum of the annular tooth portion.
Thus, the electric power steering apparatus according to the present invention is an electric power steering apparatus for steering assistance in which an output of a motor for steering assistance is transmitted to a steering shaft via a worm and a worm wheel, the worm wheel including an annular tooth portion made of synthetic resin and a boss portion made of metal, the annular tooth portion having teeth that engage with the worm, the boss portion being fitted to the inside peripheral surface of the annular tooth portion, wherein one of the inside peripheral surface of the annular tooth portion and the outside peripheral surface of the boss portion has concaves disposed on both axial ends thereof, and the other has convexes fitted into these concaves, and wherein the radial length (i.e. the length as viewed in the radial direction) of the concaves and the convexes is 1 to 2.5 times as large as the axial length (i.e. the length as viewed in the axial direction) of the concaves and the convexes.
If the radial length of the concaves and the convexes is less than 1 or more than 2.5 times as large as the axial length of the concaves and the convexes, the fatigue strength of the fit portion and the strength of the dedendum of the annular tooth portion are not enhanced so much. By setting the radial length of the concaves and the convexes to be 1 to 2.5 times as large as the axial length of the concaves and the convexes, the fatigue strength of the fit portion and the strength of the dedendum of the annular tooth portion can be further enhanced. Therefore, even if the annular tooth portion is thermally deteriorated to decrease the strength of the dedendum or absorbs moisture in the ambient atmosphere to decrease the strength of the dedendum, a strength of more than a predetermined degree can be ensured.
The above and further objects and features of the invention will more fully be apparent from the following detailed description with accompanying drawings.